Image forming apparatus

Abstract

An image forming apparatus of the type transferring a toner image from a photoconductive element to a recording medium by way of an intermediate transfer belt is disclosed. Toner images of different colors are sequentially transferred from the photoconductive element to the intermediate transfer belt one above the other, forming a full-color image. When the full-color image is to be transferred from the intermediate transfer belt to a paper or similar recording medium, a secondary transfer belt is brought into contact with the portion of the intermediate transfer belt contacting a secondary transfer roller. As a result, the secondary transfer belt deforms complementarily to the shape of the secondary transfer roller, forming a nip between it and the intermediate transfer belt. In this condition, the secondary transfer belt contacts the intermediate transfer belt evenly and prevents an excessive pressure from acting locally on the toner image existing on the intermediate transfer belt.

What is claimed is: 
     
       1. An image forming apparatus comprising: a primary transfer unit including a primary transfer section for sequentially transferring toner images of different colors from a photoconductive element to an intermediate transfer belt having a medium resistance to thereby form a composite color image on said intermediate transfer belt, and a secondary transfer roller facing an inner periphery of said intermediate transfer belt for causing said intermediate transfer belt to contact a recording medium; and   a secondary transfer unit for transferring the composite toner image from said intermediate transfer belt to the recording medium;   wherein said secondary transfer unit includes a secondary transfer belt having a medium resistance, and bias applying means facing said secondary transfer roller with said secondary transfer belt being an intermediary; and   wherein on transfer of the composite toner image from the intermediate transfer belt to the recording medium the secondary transfer belt deforms to a shape of the secondary transfer roller.   
     
     
       2. An apparatus as claimed in claim 1, wherein said bias applying means comprises a corona charger. 
     
     
       3. An apparatus as claimed in claim 2, wherein said bias applying means further comprises an inlet seal member positioned at an upstream side with respect to said secondary transfer belt to limit a charging width of the corona charger. 
     
     
       4. An image forming apparatus comprising: a primary transfer unit for sequentially transferring toner images of different colors from a photoconductive element to an intermediate transfer belt having a medium resistance to thereby form a composite color image on said intermediate transfer belt, said primary transfer unit including two secondary transfer rollers facing an inner periphery of said intermediate transfer belt and spaced by a preselected distance in a direction of transport in which a recording medium is conveyed; and   a secondary transfer unit for transferring the composite toner image from said intermediate transfer belt to the recording medium, and including a secondary transfer belt having a medium resistance, a ground roller, and bias applying means;   wherein said secondary transfer belt extends in the direction of transport, wherein said ground roller faces a portion of an inner periphery of said secondary transfer belt facing the intermediate transfer belt between said two secondary transfer rollers, and wherein said bias applying means faces the inner periphery of said secondary transfer belt at a position downstream of the downstream one of said two secondary transfer rollers with respect to the direction of transport.   
     
     
       5. An apparatus as claimed in claim 4, wherein said secondary transfer belt is capable of contacting a portion of said intermediate transfer belt extending between said two secondary transfer rollers, and wherein said secondary transfer belt forms, when contacting said portion of said intermediate transfer belt, a non-transfer region upstream of said ground roller and a transfer region downstream of said ground roller. 
     
     
       6. An apparatus as claimed in claim 4, wherein said secondary transfer belt contacts said intermediate transfer belt only during secondary image transfer from said intermediate transfer belt to the recording medium. 
     
     
       7. An image forming apparatus comprising: a primary transfer unit for sequentially transferring toner images of different colors from a photoconductive element to an intermediate transfer belt having a medium resistance to thereby form a composite color image on said intermediate transfer belt, said primary transfer unit including two secondary transfer rollers facing an inner periphery of said intermediate transfer belt and spaced by a preselected distance in a direction of transport in which a recording medium is conveyed; and   a secondary transfer unit for transferring the composite toner image from said intermediate transfer belt to the recording medium, and including a secondary transfer belt having a medium resistance, secondary transfer means, and conductive elastic means;   wherein said secondary transfer belt extends in the direction of transport, wherein said secondary transfer means faces a portion of an inner periphery of said secondary transfer belt downstream of the downstream one of said two secondary transfer rollers with respect to the direction of transport, and wherein said conductive elastic means faces upstream one of said two secondary transfer rollers with respect to the direction of transport with the intermediary of said second transfer belt, and is connected to ground.   
     
     
       8. An apparatus as claimed in claim 7, further comprising cleaning means held in contact with said secondary transfer means. 
     
     
       9. An apparatus as claimed in claim 7, wherein said conductive elastic means comprises a roller formed of a conductive foam material. 
     
     
       10. An apparatus as claimed in claim 9, further comprising cleaning means held in contact with said secondary transfer means. 
     
     
       11. An apparatus as claimed in claim 7, wherein said conductive elastic means comprises a conductive brush. 
     
     
       12. An apparatus as claimed in claim 11, further comprising cleaning means held in contact with said secondary transfer means. 
     
     
       13. An image forming apparatus comprising: a primary transfer unit for sequentially transferring toner images of different colors from a photoconductive element to an intermediate transfer belt to thereby form a composite color image on said intermediate transfer belt; and   a secondary transfer unit for transferring the composite toner image from said intermediate transfer belt to a recording medium, and including a secondary transfer belt extending in a direction of transport in which the recording medium is conveyed, and secondary transfer means facing an inner periphery of said secondary transfer belt;   a discharger and a ground member facing each other with said secondary transfer belt being an intermediary;   wherein said intermediate transfer belt is formed of a material having a volume resistivity of 10 8  Ωcm to 10 10  Ωcm, and wherein said secondary transfer belt is formed of a material having a volume resistivity of higher than 10 11  Ωcm inclusive.